This paper introduces a theoretical analysis of the performance of photovoltaic modules under different meteorological conditions and design parameters. Based on the analysis, A FORTRAN computer sub-program has been constructed and connected to TRNSYS simulation program. The present sub-program is executed within the TRNSYS program to compute the different parameters of PV modules. These parameters include short circuit current, open circuit voltage, maximum output power, I-V and P-V characteristics, and efficiency. To verify the present sub-program, an experimental set up has been installed. It includes a group of identical PV modules mounted at different tilt angles and orientations, an electronic load, a weather station, a data acquisition system, and a computer. The comparison between the experimental and theoretical results shows good agreement at different meteorological conditions, tilt angles, and orientations. (C) 2002 Elsevier Science Ltd. All rights reserved.